In a free piston apparatus of that kind, which is commonly operated in the two stroke method, the piston device oscillates back and forth in the piston receptacle. Upon the combustion of a gas-fuel-mixture in the combustion chamber, the piston is moved from a top dead center to a bottom dead center. Upon adopting the bottom dead center, the at least one inlet opening and the at least one outlet opening are opened and fresh gas is able to flow into the combustion chamber. Exhaust gas may be removed from the combustion chamber. The piston may act as a valve body with which, upon adopting the bottom dead center, the at least one inlet opening or the at least one outlet opening is at least partially unblocked and is blocked again upon the upward movement of the piston. The upward movement of the piston occurs under the action of a spring-back device of the free piston apparatus for the piston device. The spring-back device comprises for example a gas spring with a gas which is compressible by way of the piston device. Upon an expansion of the gas, the piston device is moved in the opposite direction for the upward movement of the piston. Alternatively or in addition, a mechanical spring-back device may be provided.
“Fresh gas” is presently to be understood as a gas or gas mixture (in particular air) for the internal combustion in the combustion chamber, wherein a fuel may also be admixed to the gas. “Fresh gas” may therefore presently also refer to a gas-fuel-mixture which may flow into the combustion chamber via the at least one entry opening. “Exhaust gas” presently refers to a combustion product of the internal combustion.
In the conventional free piston apparatus, a scavenging gradient is caused by the openings for the inlet and for the outlet, which are axially spaced apart from each other, and the combustion chamber is scavenged in axial direction (so-called uniflow scavenging) for the charge exchange. “Axial” and “radial” presently refer to the axis defined by the piston receptacle, along which the piston device is moved.
Free piston apparatuses may be operated in auto-ignition operation or with spark ignition, wherein an ignition device is arranged at the combustion chamber. It proves in practice to be advantageous to optimize the inflow conditions of the fresh gas into the combustion chamber with respect to the respective mixture and the spark ignition or the auto-ignition operation. For example, it is known in the case of free piston apparatuses to inflow fresh gas radially in relation to the axis for the auto-ignition operation. A helical movement of the flow forms in the combustion chamber. A flow of that kind proves to be less suitable for spark ignition operation, however.